Allow #[pymethods] to use lifetime

[kngwyu]

E.g., allow

fn method<'py>(py: Python<'py>) -> &'py PyList {...}

[codecov]

Codecov Report

Merging #461 into master will decrease coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #461      +/-   ##
==========================================
- Coverage   87.87%   87.83%   -0.04%     
==========================================
  Files          65       64       -1     
  Lines        3398     3388      -10     
==========================================
- Hits         2986     2976      -10     
  Misses        412      412

Impacted Files	Coverage Δ
src/marshal.rs

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[konstin]

Thanks you!